Device for recording of sound



May 10,1932. A; POULSEN ET AL DEVICE-FOR RECORDING OF SOUND Filed Dec. 4, 1929 j ARNOLD POULSEN, 0F HELLERUP,

Patented May 10, I932 UNITED. STATES PATENT OFFICE] ANDIAZXEL CARL enone rnrnn'sn xr, or cornnrraenn,

DENMARK I :oEvIcn non RECORDING orsoulvn Application filed December 4, 1929, Serial No.

It is commonly known to record sound on a light-sensitive film, for instance a photographic film, by causing a mirror to project a pencil of light on to the film where the said pencil is condensed and forms a .linear stripe of light, the length or intensity of which is varied in accordance with the sound oscillations.

The varying illumination of the photographic film has been effected by causing the light reflected from the mirror to oscillate across a slit through which the'film is exposed to the light. In order to attain a sharp and accurate record of the sound on the photographic film it is necessary, however,

that the portion of the film exposed atany moment should be reduced to .a very narrow stripe, the width of which, in the direction of the motion of the film, should not exceed the width of a half-wave of the highest fre-.

quency occurring in the sound record, This requirement has been filled by insertlng the said verynarrow slit directly in front of the film, or by inserting the slit at'some other. suitable point of the path of the rays of light,

and by forming then an image of the'slitin.

the plane of the fil-m by means of lenses forming, if desired, a reducedimage of tllB'Sllt.

This use of a slit, however, involves the drawback that aberration phenomena will occur in the path of the raysof light,and for this reason it has been-proposed to use, in-, stead of a narrow slit, a punctual or linear u source of light and to' form an image thereof in the plane of the film'byme'ans of a. cylin-i der lens condensing the rays of light re-.

flected from the mirror so as to form a linear image on the film. The linear source oflight may in this case be a part of an incandescent wire Y The last. mentioned process, however,

suffers from the drawbackthat an incan? descent wire oftherequired minute thickness has but a very short life,.if it has to be of a suitably high temperature enabling it to emit a suitable amount of light. Because the mirror, as a rule, has to be of very small dimensions in orderthat its mass shallnot appreciably influence its oscillations, and because the incandescent wire must not be ex- 411574, and in Denmark December 22, 1928.

posed to any especially high temperature, j only a relatively small quantity :of light will be available for lighting the film. A further drawback is that the lens which condenses said difiiculties are avoidedin that the linear member ofthe recording device is formed by the mirror itself or rather by thereflecting surface thereof, as illustrated-bythe draw ings.

Figlshows in outline a recording device; constructed according to the present inven-' tion. I p

Fig. 2 shows another form of mirror which can be used 1n our device. Y

Figs. 3 and 4 show, in perspective .vlew,

two differentconstructionsof a mirror according to the present invention. p As it appears from Fig. 1, where f is a film on to which the sound oscillations are to be recorded by means of: a mirror 8, which is caused to oscillate inaccordance with the sound oscillations and projects light from a source of light kon'tothe film f, thereis used according to the invention amirrorthewidth of which, in the direction ofmotion of the film as indicated bythe arrow injFig. 1,

is sosmall relatively to the length of the mirror in the direction across the filmthat the mirrorwill mainly be linear. zThe -word mirror means in this connection the part;

of the reflecting surface of the mirror that 1 is active in reflectingthe rays of light from the-source of light In general itwill be very diflicult to-produce a mirror which inv itself is as narrow' as desired, but a narrow reflecting surface may be procured on a broader mirror by screeningor blackening or coating so large a'portionofthe'mirror'sur face that, only .avvery narrow linear mirror band a,' Fig., 2,, in direction across the film will be active in respect to reflection.-

By the system' 'shown,;in contradistinction drical mirror, Fig. 3, the axis of which is to the heretofore used systems, an image is formed on the surface of the film, of the mirror itself, but not of an image of the source of light formed by the mirror, and therefore the lens Z, Fig. 1, serving to concentrate the light on to the mirror does not have to be a corrected lens, but may be an ordinary condenser lens of large diameter. Thereby it becomes practicable to concentrate much light on tothe mirror. The formation on the film of the image b of the mirror is effected in known manner by means of a cylinder lens 0.

In order to attain a sharpimage of the reflecting portion of the mirror the reflecting coating of the mirror, which may be for instance a mince layerof silver, is suitably applied to the front face of the mirror, and in order to prevent the layer of silver from tarnishing it may be desirable to immerse the mirror in oil.

As is well known, the image formed by a cylinder lens will be sharply defined only on two sides. In the present case the stripe of light 6 willthus only be sharply defined in the longitudinal direction of the film, while the-cylinder lens will be unable to define the image b sharply in the direction across the film. A sharp definition of the image b in the longitudinal direction of the same is, however,of importance to the purity of the sound when reproduced, and may be attained by inserting in the path of the rays, for instance in front of the lens Z, a stop as which is depicted sharply in the plane of the film' formation of such an image willgenerally' require a suitable lens to bein'serted in front of'the mirror. "The use of such a lens may be avoided, however,'according to theipresent invention" by shaping the mirror as acylinparallel to the direction of motion of the film and, therefore perpendicular to the major dimensionof themirror. Owing to its very small extent in the axial direction the refiecting surface will in. factact essentially as a concave mirror, and it will therefore form in the plane of the film a sharp image trated on the mirror 8 by way of a lens Z, which forms an image a of the linear light source on the front face of the mirror 8. The light rays reflected from the mirror 8 form an image 9 of the slit w in front of the lensZ on a screen m which is provided with a slit n. The screen m is adjusted to such position relatively to the mirror 8 that the image 9 partially appears across the slit n,for instance the upper edge of the image appears in the line i which divides the slit n longitudinally in two equal parts. The cylindrical lens 0 forms on the film f by way of the light rays passing the slit animage Z7 which image is a reconstruction of the image of the :light source lc formed on the mirror. l

It would be. obvious "that "when the mirror in known manner is caused to'oscillate about the aXiso in accordance with the sound variations, the amount of light which passes the slit n will vary and consequently theintensity of the image Z) formed on "the-film f will vary to the same degree, i. e., proportionately to the oscillationsofthe mirror and consequently in accordance with the sound amplitudes which cause the'fmirror 'to oscillate. In order-tosecure a distinctimage g a lens may be arranged in front of themirror 8.

The lightsource '70 inFig; 4 is linear. If, however, the present invention is applied :to the'arrangement according to Fig. 4,.the or-' dinary mirror 8 is replaced by a mirroriof the kind referred to aboveand shown'iniEigs. 2 or 3. Thenthe strip a (Fig 4') represents not an imageoft-heilightsource, butia linear reflecting surface, and thelight source lc'need notbe linear. i V, i p Having 'now particularly described and ascertained the nature of' our said invention and in what manner the same isi tobe performed, we declare that what we claimii'sz 1. A systeinfor recording soundson a movable film,"comprisinga light source,=axsound controlled oscillatable reflecting surface of linear dimensions,arrangedto reflecta beam of I light :from the said light source towards the film, and an optical system arranged ?to form on the filman imageofthe reflecting surface. g V Y 2. system for recording's'ounds;on amov-.1 able film, comprising a .light'fsource, a sound controlled oscillatable reflecting surface :of linear dimensions, arranged to reflect a beam of light LfI'Om :the said :light source towards the film, an opticalysystem arranged .to form on the film an image of thereflecting surface, and means for sharply defining the image formed on the film.

V Intestimony whereof we aflix our signatures.

-ARNOLD :"POULSEN. AXE'L GARL GEORG PETERSEN; 

